right now it's a generic per-template message. wouldn't it be whiz-bang cool if we could make a blog post that was for non-members only? So message-if-empty looks for that post and if not found then gives a canned per-template message?

Sure. In each template there is a bit for "message_if_empty" referring to the list of posts. The default is something like "sorry there is nothing to display". No matter how many blogs you use the same template on, the "empty message" is always the same. BTW the mainlist is empty if everything is protected, which is basically the site I'm on now.

Suppose we had a post type called "empty message". IF mainlist has nothing to show someone then it checks to see if that blog has a "empty message" post and uses it, but if not then core defaults to the simple sentence in the template. That way each blog could say a more focused message about what type of content should be there or why there is no content or whatever.